U.S. patent number 5,972,444 [Application Number 08/748,322] was granted by the patent office on 1999-10-26 for polyolefin compositions with balanced shrink properties.
This patent grant is currently assigned to The Dow Chemical Company. Invention is credited to Jacquelyn A. deGroot, Rajen M. Patel.
United States Patent |
5,972,444 |
Patel , et al. |
October 26, 1999 |
Polyolefin compositions with balanced shrink properties
Abstract
This invention relates to an improved shrink film having
balanced properties. In particular, this invention relates to a
biaxially oriented polyolefin shrink film made from a particular
polymer mixture which includes a first ethylene polymer component
having a single differential scanning calorimetry (DSC) melting
peak or a single Analytical Temperature Rising Elution
Fractionation (ATREF) peak and a second ethylene polymer component
having one or more DSC melting peaks, wherein the density
differential between the two component polymers about 0 to about
0.03 g/cc. Improved properties include increased shrink responses,
wide orientation windows, higher modulus and high softening
temperatures.
Inventors: |
Patel; Rajen M. (Lake Jackson,
TX), deGroot; Jacquelyn A. (Lake Jackson, TX) |
Assignee: |
The Dow Chemical Company
(Midland, MI)
|
Family
ID: |
27555707 |
Appl.
No.: |
08/748,322 |
Filed: |
November 13, 1996 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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428273 |
Apr 25, 1995 |
5632510 |
|
|
|
055063 |
Apr 28, 1993 |
5562958 |
|
|
|
916269 |
Jul 21, 1992 |
5296175 |
|
|
|
024563 |
Mar 1, 1993 |
|
|
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|
776130 |
Oct 15, 1991 |
5272376 |
|
|
|
Current U.S.
Class: |
428/35.2;
428/218; 525/240; 428/516; 428/36.9; 428/349 |
Current CPC
Class: |
B32B
27/08 (20130101); B32B 27/32 (20130101); B32B
27/322 (20130101); B42D 15/00 (20130101); C08F
10/00 (20130101); C08F 10/02 (20130101); C08F
210/16 (20130101); C08G 83/003 (20130101); C08J
5/18 (20130101); C08L 23/04 (20130101); C08L
23/0815 (20130101); C08L 23/16 (20130101); B29C
48/15 (20190201); B29C 48/022 (20190201); B29C
48/08 (20190201); C08F 10/00 (20130101); C08F
4/6592 (20130101); C08F 210/16 (20130101); C08F
4/6592 (20130101); C08L 23/04 (20130101); C08L
23/0815 (20130101); C08L 23/16 (20130101); B29K
2995/0022 (20130101); C08F 4/65908 (20130101); C08F
4/65912 (20130101); C08F 4/6592 (20130101); C08F
110/00 (20130101); C08F 110/02 (20130101); C08J
2303/08 (20130101); C08J 2323/08 (20130101); C08L
23/02 (20130101); C08L 2205/02 (20130101); Y10T
428/1334 (20150115); B29C 48/00 (20190201); B29C
48/13 (20190201); Y10T 428/24992 (20150115); Y10T
428/31913 (20150401); Y10T 428/139 (20150115); Y10T
428/2826 (20150115); C08L 2666/04 (20130101); C08L
2666/04 (20130101); C08L 2666/04 (20130101); C08F
110/02 (20130101); C08F 2500/12 (20130101); C08F
2500/07 (20130101); C08F 2500/03 (20130101); C08F
2500/19 (20130101); C08F 2500/17 (20130101); C08F
110/02 (20130101); C08F 2500/08 (20130101); C08F
2500/09 (20130101); C08F 2500/11 (20130101); C08F
2500/12 (20130101); C08F 2500/05 (20130101); C08F
110/02 (20130101); C08F 2500/12 (20130101); C08F
2500/19 (20130101); C08F 2500/11 (20130101); C08F
210/16 (20130101); C08F 210/14 (20130101); C08F
2500/26 (20130101); C08F 210/16 (20130101); C08F
210/14 (20130101); C08F 2500/12 (20130101); C08F
2500/17 (20130101); C08F 2500/09 (20130101); C08F
2500/03 (20130101); C08F 2500/26 (20130101); C08F
210/16 (20130101); C08F 210/14 (20130101); C08F
2500/12 (20130101); C08F 2500/19 (20130101); C08F
2500/11 (20130101); C08F 210/16 (20130101); C08F
210/14 (20130101); C08F 2500/03 (20130101); C08F
2500/09 (20130101); C08F 2500/08 (20130101); C08F
2500/12 (20130101); C08F 2500/26 (20130101); C08F
210/16 (20130101); C08F 210/14 (20130101); C08F
2500/08 (20130101); C08F 2500/11 (20130101); C08F
2500/03 (20130101); C08F 2500/09 (20130101); C08F
210/16 (20130101); C08F 210/14 (20130101); C08F
2500/19 (20130101); C08F 2500/12 (20130101); C08F
2500/17 (20130101); C08F 2500/09 (20130101); C08F
2500/03 (20130101) |
Current International
Class: |
B42D
15/00 (20060101); B29C 47/00 (20060101); C08G
83/00 (20060101); B29C 47/02 (20060101); B32B
27/08 (20060101); B32B 27/32 (20060101); C08F
10/00 (20060101); C08F 10/02 (20060101); C08J
5/18 (20060101); C08F 210/00 (20060101); C08L
23/04 (20060101); C08L 23/00 (20060101); C08L
23/08 (20060101); C08L 23/16 (20060101); C08F
210/16 (20060101); C08F 4/00 (20060101); C08F
110/00 (20060101); C08F 4/659 (20060101); C08F
110/02 (20060101); C08F 4/6592 (20060101); C08L
23/02 (20060101); B32B 007/02 (); B32B
027/08 () |
Field of
Search: |
;525/240
;428/218,349,516,35.2,36.91 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
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|
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0 436 196 A2 |
|
Jul 1991 |
|
EP |
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0 572 034 A2 |
|
Mar 1993 |
|
EP |
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0 737 713 A1 |
|
Sep 1996 |
|
EP |
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94/25523 |
|
Jun 1994 |
|
WO |
|
95/10566 |
|
Apr 1995 |
|
WO |
|
96/12762 |
|
Jun 1996 |
|
WO |
|
Other References
Kissin, "Olefin Polymers", Enc. Chem. Tech., vol. 17, p. 704
(1996)..
|
Primary Examiner: Wilson; Donald R.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
(This is a continuation-in-part of application Ser. No. 08/428,273,
filed Apr. 25, 1995, now U.S. Pat. No. 5,632,510, which is a
division of application Ser. No. 08/055,063, filed Apr. 28, 1993,
now U.S. Pat. No. 5,562,958, which is a continuation-in-part of
application Ser. No. 07/916,269, filed Jul. 21, 1992, now U.S. Pat.
No. 5,296,175, and a continuation-in-part of application Ser.
No.08/024,563, filed Mar. 1, 1993, now abandoned, which is a
continuation in part of application Ser. No. 07/776,130, filed Oct.
15, 1991, now U.S. Pat. No. 5,272,376.) This application also
claims the benefits of provisional application Ser. No. 60/011,874,
filed Feb. 20, 1996. The disclosure of all of the above
applications are incorporated herein by reference.
Claims
We claim:
1. A multilayer shrink film having a shrink control layer, the
shrink control layer comprising a polymer mixture wherein the
polymer mixture has a density in the range of about 0.88
gram/centimeter (g/cc) to about 0.94 g/cc as determined in
accordance with ASTM D-792 and includes
(A) from about 20 to about 80 weight percent, based on the total
weight of the polymer mixture, of at least one first ethylene
polymer which is a substantially linear ethylene/.alpha.-olefin
interpolymer containing at least one C.sub.3 -C.sub.20
.alpha.-olefin, having from about 0.01 to 3 long chain branches per
1000 carbons and characterized as having:
i. a short chain branching distribution index (SCBDI) greater than
50 percent,
ii. a single melting peak between -30.degree. and 150.degree. C. as
determined using differential scanning calorimetry (DSC),
iii. a melt flow ratio, I.sub.10 /I.sub.2, equal to or greater than
5.63,
iv. a molecular weight distribution, M.sub.w /M.sub.n, defined by
the equation:
v. a gas extrusion rheology critical shear rate wherein the
critical shear rate at the onset of surface melt fracture for the
substantially linear ethylene/.alpha.-olefin polymer is at least 50
percent greater than the critical shear rate at the onset of
surface melt fracture for a linear ethylene/.alpha.-olefin polymer
containing at least one C.sub.3 -C.sub.20 .alpha.-olefin, wherein
the linear ethylene/.alpha.-olefin polymer is characterized as
having an I.sub.2, and M.sub.w /M.sub.n within ten percent of the
substantially linear ethylene/.alpha.-olefin polymer, and wherein
the critical shear rates of the substantially linear
ethylene/.alpha.-olefin polymer and the linear
ethylene/.alpha.-olefin polymer are measured at the same melt
temperature using a gas extrusion rheometer, and
vi. a density in the range of about 0.87 (g/cc) to about 0.93 g/cc
as determined in accordance with ASTM D-792, and
(B) from about 20 to about 80 weight percent, based on the total
weight of the polymer mixture, of at least one second ethylene
polymer which is a heterogeneously branched ethylene/.alpha.-olefin
ethylene interpolymer containing at least one C.sub.3 -C.sub.20
.alpha.-olefin characterized as having one or more melting peaks as
determined using differential scanning calorimetry (DSC) and a
density in the range of about 0.89 (g/cc) to about 0.96 g/cc as
determined in accordance with ASTM D-79,
wherein the density differential between the first and second
ethylene polymer components, determined in accordance with ASTM
D-792, is in the range of from about 0 to about 0.025 g/cc and the
density of the first ethylene polymer is equal to or lower than the
density of the second ethylene polymer.
2. The shrink film of claim 1 wherein the multilayer film structure
is prepared by a coextrusion technique.
3. The shrink film of claim 1 wherein the multilayer film structure
is prepared by a lamination technique.
4. The shrink film of claim 1, wherein the multilayer film
structure includes a sealant layer comprising at least one
homogeneously branched ethylene/.alpha.-olefin interpolymer.
5. The multilayer shrink film of claim 1, wherein the at least one
first ethylene polymer is a copolymer of ethylene and 1-octene.
6. The multilayer shrink film of claim 1, wherein the density
differential is in the range of from about 0.015 to about 0.025
g/cc.
7. The multilayer shrink film of claim 1, wherein the at least one
first ethylene polymer component (A) and the at least one second
ethylene polymer component (B) are further characterized as having
a DSC crystallinity in weight percent where the DSC crystallinity
differential between the at least one first ethylene polymer
component (A) and the at least one second ethylene polymer
component (B) is from about 7 to about 21%.
8. The multilayer shrink film of claim 1, wherein the film is a
biaxially oriented shrink film having free shrinkage in the machine
and transverse directions.
Description
FIELD OF THE INVENTION
This invention relates to an improved shrink film. In particular,
this invention relates to a biaxially oriented polyolefin shrink
film made from a polymer mixture comprised of (A) a first ethylene
polymer component having a single differential scanning calorimetry
(DSC) melting peak and a single Analytical Temperature Rising
Elution Fractionation (ATREF) peak or (B) a second ethylene polymer
component having one or more DSC melting peaks, wherein the density
differential between component(A) and component (B) is in the range
from about 0 to about 0.03 g/cc.
BACKGROUND OF THE INVENTION
Food items such as poultry, fresh red meat and cheese, as well as
nonfood industrial and retail goods, are packaged by various heat
shrink film methods. There are two main categories of heat shrink
films--hot-blown shrink film and oriented shrink film. Hot-blown
shrink film is made by a hot-blown simple bubble film process and
oriented shrink film is made by elaborate processes known as double
bubble, tape bubble, trapped bubble or tenter framing. Heat shrink
films can be monoaxial or biaxial oriented and are required to
possess various other film attributes. In addition to a high shrink
response, for successful use in hot-fill or cook-in applications,
shrink films must also possess a relatively high softening
point.
The shrink packaging method generally involves placing an
article(s) into a bag (or sleeve) fabricated from a heat shrink
film, then closing or heat sealing the bag, and thereafter exposing
the bag to sufficient heat to cause shrinking of the bag and
intimate contact between the bag and article. The heat can be
provided by conventional heat sources, such as heated air, infrared
radiation, hot water, combustion flames, or the like. Heat shrink
wrapping of food articles helps preserve freshness, is attractive,
hygienic, and allows closer inspection of the quality of the
packaged food. Heat shrink wrapping of industrial and retail goods,
which is alternatively referred to in the art and herein as
industrial and retail bundling, preserves product cleanliness and
also is a convenient means of bundling and collating for accounting
and transporting purposes.
The biaxial heat-shrink response of an oriented polyolefin film is
obtained by initially stretching fabricated film to an extent
several times its original dimensions in both the machine and
transverse directions to orient the film. The stretching is usually
accomplished while the fabricated film is sufficiently soft or
molten, although cold drawn shrink films are also known in the art.
After the fabricated film is stretched and while still in a
stretched condition, the stretch orientation is frozen or set in by
quick quenching of the film. Subsequent application of heat will
then cause the oriented film to relax and, depending on the actual
shrink temperature, the oriented film can return essentially back
to its original unstretched dimensions, i.e., to shrink relative to
its stretched dimension.
Hence, clearly the orientation window and shrink response of
oriented films affected by resin properties and fabrication
parameters. The orientation window depends upon the broadness of
the resin melting range and, as such, relates directly to the short
chain branching distribution of the resin. In general, ethylene
alpha-olefin interpolymers having a broad short chain branching
distribution and broad melting range (e.g., heterogeneously
branched ultra low density polyethylene resins such as ATTANE.TM.
resins supplied by The Dow Chemical Company) exhibit a wider
orientation window compared to ethylene alpha-olefin interpolymers
characterized as having a narrow short chain branching distribution
and narrow melting range (e.g., homogeneously branched linear
ethylene polymers such as EXCEED.TM. and EXACT.TM. resins supplied
by Exxon Chemical Corporation).
Oriented polyolefin film shrinkage depends on shrink tension and
film density. Film shrinkage is decreased as the orientation
temperature is increased due to lower shrink tension. Film
shrinkage is increased at lower density (lower crystallinity)
because crystallites provide topological constraints and, as such,
hinder free shrinkage. Conversely, for given draw ratio, shrink
tension depends on the crystallinity of the resin at the
orientation temperature.
While the temperature at which a particular polymer is sufficiently
soft or molten is a critical factor in various orientation
techniques, in general, such temperatures are ill-defined in the
art. Disclosures pertaining to oriented films that disclose various
polymer types (which invariably have varying polymer
crystallinities and melting points), simply do not define the
stretching or orientation temperatures used for the reported
comparisons. U.S. Pat. No. 4,863,769 to Lustig et al., WO 95/00333
to Eckstein et al., and WO 94/07954 to Garza et al., the
disclosures of which are incorporated herein by reference, are two
examples of such disclosures.
The direct effect of density or crystallinity on shrink response
and other desired shrink film properties such as, for example,
impact resistance, are known, for example, from WO 95/08441, the
disclosure of which is incorporated herein by reference. That is,
even where the orientation temperature is presumably constant,
lower density polymer films will show a higher shrink response and
improved impact resistance. However, the effects of density and
other resin properties on the orientation temperature is not
well-known. In the prior art, there are only general rules of thumb
or generalized teachings relating to suitable stretching or
orientation conditions. For example, in commercial operations, it
is often said that the temperature at which the film is suitably
soft or molten is just above its respective glass transition
temperature, in the case of amorphous polymers, or below its
respective melting point, in the case of semi-crystalline
polymers.
While the effects of density and other resin properties on the
optimum orientation temperature of polyolefins are generally
unknown, it is clear that heterogeneously branched ethylene
polymers such as ATTANE.TM. resins and DOWLEX.TM. resin have a
relatively broad orientation window (i.e., the temperature range at
which the resin can be substantially stretched when molten or
softened). It also clear that softening temperatures and other film
properties such as, for example, secant modulus, tend to decrease
at lower polymer densities. Because of these relationships, films
with high shrink responses, wide orientation windows, high modulus
and high softening temperatures (i.e., shrink films with balanced
properties) are unknown in the prior art. That is, polymer
designers invariably have to sacrifice high softening temperatures
and high modulus to provide films with high shrink responses and
wide orientation windows. The importance of higher modulus pertains
to, for example, the need for good machinability during automatic
packaging operations and good handling during bag making
operations.
An example of teaching that's beyond ordinary rules of thumb (but
is nevertheless fairly generalized) is provided by Golike in U.S.
Pat. No. 4,597,920, the disclosure of which is incorporated herein
by reference. Golike teaches orientation should be carried out at
temperatures between the lower and higher melting points of a
copolymer of ethylene with at least one C.sub.8 -C.sub.18
.alpha.-olefin. Golike specifically teaches that the temperature
differential is at least 10.degree. C., however, Golike also
specifically discloses that the full range of the temperature
differential may not be practical because, depending on the
particular equipment and technique used, tearing of the polymer
film may occur at the lower end of the range. At the higher limit
of the range, Golike teaches the structural integrity of the
polymer film begins to suffer during stretching (and ultimately
fails at higher temperatures) because the polymer film then is in a
soft, molten condition. See, U.S. Pat. No. 4,597,920, Col. 4, lines
52-68 bridging to Col. 5., lines 1-6. The orientation temperature
range defined by Golike (which is based on higher and lower peak
melting points) generally applies to polymer blends and
heterogeneously branched ethylene/.alpha.-olefin interpolymers,
i.e. compositions having two or more DSC melting points, and does
not apply at all to homogeneously branched ethylene/.alpha.-olefin
interpolymers which have only a single DSC melting point. Golike
also indicates that a person of ordinary skill can determine the
tear temperature of a particular polymer and discloses that for
heterogeneously branched interpolymers having a density of about
0.920 g/ cc, the tear temperature occurs at a temperature above the
lower peak melting point. See, U.S. Pat. No. 4,597,920, Col. 7,
Example 4. However, Golike does not teach or suggest how a person
of ordinary skill in the art of shrink film can optimize the
orientation process as to stretching temperature at a given
stretching rate and ratio to maximize the shrink response and
achieve balanced properties.
Hideo et al. in EP 0359907 A2, the disclosure of which is
incorporated herein by reference, teach the film surface
temperature at the starting point of stretching should be within
the range of from 20.degree. C. to about 30.degree. C. below the
melting temperature of the polymer as determined in regards to the
main DSC endothermic peak. While such teaching is considered
applicable to homogeneously branched ethylene/.alpha.-olefin
interpolymers having a single DSC melting peak, the prescribed
range is fairly general and broad. Moreover, Hideo et al. do not
provide any specific teaching as to the optimum orientation
temperature for a particular interpolymer respecting heat shrink
response, nor any other desired shrink film property.
WO 95/08441, the disclosure of which is incorporated herein by
reference, provides generalized teachings pertaining to
homogeneously branched ethylene/cc-olefin interpolymers. In the
Examples of this disclosure, several different homogeneously
branched substantially linear ethylene/.alpha.-olefin interpolymers
were studied and compared to one heterogeneously branched ethylene/
.alpha.-olefin interpolymers. Although the homogeneously branched
substantially linear ethylene/.alpha.-olefin interpolymers had
densities that varied from about 0.896 to about 0.906 g/cc, all of
the interpolymers (including the heterogeneously branched linear
ethylene/.alpha.-olefin interpolymer, ATTANE.TM. 4203, supplied by
The Dow Chemical Company, which had a density of 0.905 g/cc) were
oriented at essentially the same orientation temperatures. Reported
results in WO 95/08441 disclose three general findings: (1) at an
equivalent polymer density, substantially linear
ethylene/.alpha.-olefin interpolymers and heterogeneously branched
linear ethylene/cc-olefin interpolymers have essentially equivalent
shrink responses (compare Example 21 and Example 39 at pages
15-16), (2) shrink responses increase at lower densities and
constant orientation temperatures, and (3) as orientation
temperature increases, orientation rates increase. Furthermore,
careful study of the Examples and unreported DSC melting point data
for the interpolymers reported on in WO 95/08441 indicate for the
Examples disclosed in WO 95/08441 that, at a given stretching rate
and ratio, there is a preference for orienting multilayer film
structures at orientation temperatures above the respective DSC
melting point of the polymer employed as the shrink control layer.
Moreover, none of the teachings or Examples in WO 95/08441 suggest
a shrink film with balanced properties is obtainable.
Other disclosures that set forth orientation information regarding
homogeneously branched ethylene polymers yet do not specify
orientation conditions relative to respective lowest stretch
temperatures, nor teach requirements for balanced shrink film
properties include EP 0 600425A1 to Babrowicz et al. and EP 0
587502 A2 to Babrowicz et al., the disclosures of which are
incorporated herein by reference.
Accordingly, although there are general rules and general
disclosures as to shrink responses and suitable orientation
temperatures for biaxially orienting polyolefins, there is no
specific information as to optimum orientation conditions as a
function of polymer type and, more importantly, there is no
specific information as to balanced or optimized shrink responses,
wide orientation windows, high modulus and high softening
temperatures. As such, it is an object of the present invention to
provide an improved shrink film with a maximized shrink response,
an increased orientation window and, for a given modulus or polymer
density, a relatively high softening temperature . This and other
objects will become apparent from the description and various that
follow.
SUMMARY OF THE INVENTION
In accordance with the present invention, we have discovered that
for polymer mixtures comprised of at least two ethylene polymers,
when the density differential between the component polymers is
selectively controlled and optimized, a substantially improved
shrink film is obtained. The improved shrink film will have a high
shrink response, a wide orientation window and a relatively high
softening temperature.
A broad aspect of the present invention is a shrink film comprising
a polymer mixture wherein the polymer mixture has a density in the
range of about 0.88 gram/centimeter (g/cc) to about 0.94 g/cc as
determined in accordance with ASTM D-792 and includes
(A) from about 20 to about 80 weight percent, based on the total
weight of the polymer mixture, of at least one first ethylene
polymer characterized as having a single melting peak as determined
using differential scanning calorimetry (DSC) or a single
Analytical Temperature Rising Elution Fractionation (ATREF) peak
and a density in the range of about 0.87 (g/cc) to about 0.93 g/cc
as determined in accordance with ASTM D-792, and
(B) from about 20 to about 80 weight percent, based on the total
weight of the polymer mixture, of at least one second ethylene
polymer characterized as having one or more melting peaks as
determined using differential scanning calorimetry (DSC) and a
density in the range of about 0.89 (g/cc) to about 0.96 g/cc as
determined in accordance with ASTM D-79,
wherein the density differential between the first and second
ethylene polymer components, determined in accordance with ASTM
D-792, is in the range of from about 0 to about 0.03 g/cc and the
molecular weight of the at least one first ethylene polymer is
higher than the molecular weight of the least one second ethylene
polymer.
Unexpectedly, the present inventive shrink film shows an improved
shrink response at a comparatively higher density while typically
lower densities are required for such improvement. As another
unexpected surprise, the inventive shrink film also shows a
comparatively high softening temperature for its given shrink
response where typically for ethylene alpha-olefin interpolymer
softening temperatures are reduced where the shrink response is
improved. Stated differently, the inventive shrink film exhibits
surprisingly higher shrinkage at equivalent or higher softening
temperature whereas for prior art materials, softening temperatures
must be decreased for higher shrinkage performance.
While the present invention allows practitioners to realize
increased unrestrained shrink performance, the benefits of this
invention are particularly useful for those common commercial
instances where the orientation temperature capabilities of the
stretching operation are essentially fixed. That is, by providing
an increased orientation window, a film composition that could not
be successfully stretched at all within a given equipment
capability can now be conveniently oriented.
BRIEF DESCRIPTION OF DRAWING
FIG. 1 is an Analytical Temperature Rising Elution Fractionation
curve of an ethylene polymer having a single ATREF peak.
DEFINITIONS OF TERMS
The term "polymer", as used herein, refers to a polymeric compound
prepared by polymerizing monomers, whether of the same or a
different type. The generic term "polymer" thus embraces the terms
"homopolymer," "copolymer," "terpolymer" as well as
"interpolymer."
The term "interpolymer", as used herein, refers to polymers
prepared by the polymerization of at least two different types of
monomers. The generic term "interpolymer" thus includes the term
"copolymers" (which is usually employed to refer to polymers
prepared from two different monomers) as well as the term
"terpolymers" (which is usually employed to refer to polymers
prepared from three different types of monomers).
"Stretched" and "oriented" are used in the art and herein
interchangeably, although orientation is actually the consequence
of a film being stretched by, for example, internal air pressure
pushing on the tube or by a tenter frame pulling on the edges of
the film.
The term "lowest stretch temperature" as used herein means the
temperature below which the film either tears and/or stretches
unevenly for a given stretching rate and stretching (draw) ratio
during the stretching operation or step of an orientation
technique. The lowest stretch temperature is (1) below the melting
point of the film, (2) a temperature below which the film can not
be uniformly stretched (i.e., without the occurrence of banding or
necking or the sample dislodging from the grips of the stretcher at
grip pressures of about 500 psi), and (3) a temperature below which
the film tears for a particular stretching rate and stretch
ratio.
Practitioners will appreciate that to maximize the stretch imparted
and therefore the shrink response, the objective is to operate as
close to the lowest stretch temperature as their equipment and
capabilities will allow whether or not the significant stretching
or orientation is accomplished in a single step or by a combination
of sequential steps.
Additionally, practitioners will appreciate that the optimum or
near-optimum stretching temperature for maximized shrink response
at a given shrink temperature will interrelate with stretching rate
and ratio. That is, while a particular stretching temperature will
be optimum or near-optimum at one combination of stretching rate
and ratio, the same stretching temperature will not be optimum or
near-optimum at a different combination of stretching rate and
ratio.
Practitioners will also appreciate that to obtain the maximum
shrink response from the orientation frozen into the film, the
shrink temperature should match or exceed the stretching
temperature. That is, reduced shrink temperatures do not allow full
relaxation or shrinkage of the film. However, excessive shrink
temperatures can diminish film integrity.
Practitioners will further appreciate that for a given combination
of stretching temperature, stretching rate and stretching ratio,
increases in the shrink temperature to the point of film integrity
failure will yield higher shrink response performance and higher
levels of shrink tension.
Shrink temperatures in the range of from about 70 to about
140.degree. C., especially from about 80 to about 125.degree. C.,
and more especially from about 85 to about 110.degree. C. are
suitable in the present invention.
The term "residual crystallinity" is used herein to refer the
crystallinity of a polymer film at a particular stretching
temperature. Residual crystallinity is determined using a
Perkin-Elmer DSC 7 set for a first heat at 10.degree. C./min. of a
water-quenched, compression molded film sample of the polymer. The
residual crystallinity for an interpolymer at a particular
temperature is determined by measuring heat of fusion between that
temperature and the temperature of complete melting using a partial
area technique and by dividing the heat of fusion by 292 Joules/
gram. The heat of fusion is determined by computer integration of
the partial area using Perkin-Elmer PC Series Software Version
3.1.
The term "shrink control layer" is used herein to refer to the film
layer that provides or controls the shrink response. Such a layer
is inherent to all heat shrink films. In a monolayer heat shrink
film, the shrink control layer will be the film itself. In a
multilayer heat shrink film, the shrink control layer is typically
the core or an inside film layer and is typically the thickest film
layer. See, for example, WO 95/08441.
The term "substantially unoriented form" is used herein in
reference the fact that some amount of orientation is usually
imparted to a film during ordinary fabrication. As such, it is
meant that the fabrication step, in itself, is not used to impart
the degree of orientation required for the desired or required
shrink response. The present invention is thought to be generally
applicable to operations where the fabrication and orientation
steps are separable and occur simultaneously. However, the present
invention is preferably directed to an additional and separate
orientation step which is required beyond the making of tube, sock,
web or layflat sheet whether or not such is soft, molten, or
irradiated before substantial orientation is imparted.
DETAILED DESCRIPTION OF THE INVENTION
Double bubble and trapped bubble biaxial orientation methods can be
simulated on a laboratory scale using a T. M. Long stretcher which
is analogous to a tenter frame device. This device can orient
polyolefin films in both the monoaxial and biaxial mode at
stretching ratios up to at least 5:1. The device uses films having
an original dimension of 2 inches.times.2 inches. Biaxial
stretching is usually performed by stretching in the machine
direction and transverse direction of the film simultaneously,
although the device can be operated to stretch sequentially.
The residual crystallinity of polyolefin interpolymers (measured
using a DSC partial area method) can be used to characterize the
nature of polyolefin film at the orientation temperature. In
general, it preferred to orient polyolefin films at a an
orientation temperature where the residual crystallinity of the
film is as high as possible. Such an orientation will generally be
only a few degree above that temperature where the film can no
longer be successfully oriented. That is, about 5.degree. C. above,
preferably about 3.degree. C. above, more preferably about
2.5.degree. C. above the lowest stretch temperature (defined herein
above) is considered herein to be the optimum or near-optimum
stretching or orientation temperature for the particular film.
Stretching temperatures less than about 2.5.degree. C. above the
lowest stretch temperature are not preferred because they tend to
yield inconsistent results, although such inconsistencies tend to
depend on specific equipment and temperature control capabilities.
However, for proper comparison of various films, an orientation
temperature should be selected such that the residual crystallinity
at orientation is approximately the same for each film. That is,
although wide orientation windows are desired, selection of the
actual orientation temperature to be employed should never be
arbitrary.
The density differential between the at least one first ethylene
polymer component (A) and the at least one second ethylene polymer
component (B) is generally in the range of from about 0 to about
0.03 g/cc, preferably in the range of from about 0.01 to about 0.03
g/ cc, more preferably in the range of from about 0.015 to about
0.025 g/cc, as measured in accordance with ASTM D-792. A percent
DSC crystallinity may also be used to characterize the at least one
first ethylene polymer component and the at least one second
ethylene polymer component. That is, the percent DSC crystallinity
differential between the at least one first ethylene polymer
component (A) and the at least one second ethylene polymer
component (B) is generally in the range of from about 0 to about
23%, preferably in the range of from about 7 to about 20%, more
preferably in the range of from about 10 to about 18%.
The first ethylene polymer component (A) has a density in the range
of from about 0.87 to about 0.93 g/cc, preferably from about 0.88
to about 0.92 g/cc (as measured in accordance with ASTM D-792). The
second ethylene polymer component (B) has a density in the range of
from about 0.89 to about 0.96 g/cc, preferably from about 0.90 to
about 0.94 g/cc (as measured in accordance with ASTM D-792).
Additionally, it is preferred that the density of the at least one
first ethylene polymer component (A) is lower than the density of
the at least one second ethylene polymer component (B).
The overall density of the polymer mixture (i.e., the combination
of component (A) and component (B) is generally in the range of
from about 0.88 to about 0.94 g/cc, preferably in the range of from
about 0.89 to about 0.93 g/cc, more preferably in the range of from
about 0.90 to about 0.93 g/cc, and most preferably in the range of
from about 0.90 to about 0.92 g/cc (as measured in accordance with
ASTM D-792).
The first ethylene polymer component of the polymer mixture used in
the invention, Component (A), is at least one ethylene polymer
having a single DSC melting peak or a single ATREF peak. By single
ATREF peak, it is meant that the purge portion or
non-crystallizable polymer fraction observed in a typical ATREF
curve is not considered to be an ATREF peak. For example, in FIG.
1, the elution at the elution temperature of about 20.degree. C. is
a purge portion and not an ATREF peak. As such, the polymer is
characterized as having a single ATREF peak which peaks at an
elution temperature of about 57.5.degree. C. Suitable polymers for
use as the at least one first ethylene polymer, include
homogeneously branched substantially linear ethylene polymers and
homogeneously branched linear ethylene polymers.
The second component polymer of the polymer mixture is at least one
ethylene polymer having one or more DSC melting peaks. Suitable
polymers for use as the at least one second ethylene polymer
include heterogeneously branched linear low density polyethylene
(e.g., linear low density polyethylene and ultra or very low
density polyethylene), substantially linear ethylene polymers,
homogeneously branched linear ethylene polymers, high pressure
ethylene polymers (e.g., low density polyethylene, ethylene vinyl
acetate (EVA) copolymer, ethylene acrylic acid (EAA) copolymer or
ethylene methacrylic acid (EMAA) ionomer) and combinations or
mixtures thereof.
However, preferably the first ethylene polymer component (A) is at
least one substantially linear ethylene polymer and the second
component polymer (B) is a heterogeneously branched linear ethylene
polymer. Substantially linear ethylene polymers are preferred as
the first ethylene polymer component (A) due to their improved melt
extrusion processability and unique Theological properties as
described by Lai et. al in U.S. Pat. No. Nos. 5,272,236 and
5,278,272, the disclosures of which are incorporated herein by
reference.
The molecular weight of polyolefin polymers is conveniently
indicated using a melt index measurement according to ASTM D-1238,
Condition 190.degree. C./2.16 kg (formerly known as "Condition E"
and also known as I.sub.2). Melt index is inversely proportional to
the molecular weight of the polymer. Thus, the higher the molecular
weight, the lower the melt index, although the relationship is not
linear. Component (A) and component (B) will be independently
characterized by an I.sub.2 melt index with the at least one first
ethylene polymer having a higher molecular weight than the at least
one second ethylene polymer. By "independently characterized" it is
meant that the I.sub.2 melt index of component (A) need not be the
same as the I.sub.2 melt index of component (B).
The first ethylene polymer component (A) has an I.sub.2 melt index
in the range of from greater than or equal to about 0.01 g/10
minutes to less than or equal to about 50 g/10 minutes, preferably
from greater than or equal to about 0.05 g/10 minutes to less than
or equal to about 20 g/10 minutes, most preferably from greater
than or equal to about 0.5 g/10 minutes to less than or equal to
about 10 g/10 minutes.
The second ethylene polymer component (B) may have an I.sub.2 melt
index in the range of from about 0.01 g/10 minutes to about 100
g/10 minutes, preferably from about 0.05 g/10 minutes to 50 g/10
minutes, more preferably from about 0.1 g/10 minutes to about 20
g/10 minutes, and most preferably from about 0.5 g/10 minutes to
about 10 g/10 minutes.
The overall melt index of the polymer mixture is preferably in the
range of from about 0.1 to about 5 g/10 minutes, more preferably
from about 0.5 to about 4 g/10 minutes.
Other measurements useful in characterizing the molecular weight of
substantially linear ethylene interpolymers and homopolymers
involve melt index determinations with higher weights, such as, for
common example, ASTM D-1238, Condition 190.degree. C./10 kg
(formerly known as "Condition N" and also known as I.sub.10). The
ratio of a higher weight melt index determination to a lower weight
determination is known as a melt flow ratio, and for measured
I.sub.10 and the I.sub.2 melt index values the melt flow ratio is
conveniently designated as I.sub.10 /I.sub.2. For the substantially
linear ethylene polymers used to prepare the films of the present
invention, the melt flow ratio indicates the degree of long chain
branching, i.e., the higher the I.sub.10 /I.sub.2 melt flow ratio,
the more long chain branching in the polymer. In addition to being
indicative of more long chain branching, higher I.sub.10 /I.sub.2
ratios are also indicative of lower viscosity at higher shear rates
(easier processing) and higher extensional viscosity.
In general, the at least one first ethylene polymer component (A)
has an I.sub.10 /I.sub.2 melt flow ratio greater than about 6,
preferably from greater than about 7, more preferably greater than
8, and most preferably in the range of from about 8.5 to about 20.
Embodiments that meet the specified density differential and have
an I.sub.10 /I.sub.2 melt flow ratio greater than about 8 are
particularly preferred embodiments of the present invention.
The first ethylene polymer component (A) generally constitutes from
about 20 to about 80 weight percent of the polymer mixture, based
on the total weight of the polymer mixture and preferably from
about 30 to about 70 weight percent of the polymer mixture, based
on the total weight of the polymer mixture. Conversely, the polymer
mixture used in the present invention comprises from about 20 to
about 80 weight percent and preferably from about 30 to 70 weight
percent of the at least one second ethylene polymer component (B),
based on the total weight of the polymer mixture.
Suitable ethylene polymers for use as the second component polymer
(B) include substantially linear ethylene interpolymers,
homogeneously branched linear ethylene interpolymers,
heterogeneously branched linear ethylene interpolymers (e.g.,
linear low density polyethylene (LLDPE), medium density
polyethylene (MDPE), high density polyethylene (HDPE) and ultra low
or very low density polyethylene (ULDPE or VLDPE)), and
combinations or mixtures thereof.
Substantially linear ethylene polymers are sold under the
designation of AFFINITY.TM. and ENGAGE.TM. resins by The Dow
Chemical Company and Dupont Dow Elastomers, respectively.
Homogeneously branched linear ethylene polymers are sold under the
designation of TAFMER.TM. by Mitsui Chemical Corporation and under
the designation of EXACT.TM. and EXCEED.TM. resins by Exxon
Chemical Corporation, respectively. Heterogeneously branched linear
ethylene polymers are sold under the designations of ATTANE.TM. and
DOWLEX.TM. by The Dow Chemical Company and under the designation of
FLEXOMER by Union Carbide Corporation.
The term "homogeneously branched linear ethylene polymer" is used
in the conventional sense in reference to a linear ethylene
interpolymer in which the comonomer is randomly distributed within
a given polymer molecule and wherein substantially all of the
polymer molecules have the same ethylene to comonomer molar ratio.
The terms refer to an ethylene interpolymer that is characterized
by a relatively high short chain branching distribution index
(SCBDI) or composition distribution branching index (CDBI). That
is, the interpolymer has a SCBDI greater than or equal to about 50
percent, preferably greater than or equal to about 70 percent, more
preferably greater than or equal to about 90 percent and
essentially lack a measurable high density (crystalline) polymer
fraction in TREF analysis.
SCBDI is defined as the weight percent of the polymer molecules
having a comonomer content within 50 percent of the median total
molar comonomer content and represents a comparison of the monomer
distribution in the interpolymer to the monomer distribution
expected for a Bernoullian distribution. The SCBDI of an
interpolymer can be readily calculated from data obtained from
techniques known in the art, such as, for example, temperature
rising elution fractionation (abbreviated herein as "TREF") as
described, for example, by Wild et al., Journal of Polymer Science,
Poly. Phys. Ed., Vol. 20, p. 441 (1982), or in U.S. Pat. Nos.
4,798,081; 5,008,204; or by L. D. Cady, "The Role of Comonomer Type
and Distribution in LLDPE Product Performance," SPE Regional
Technical Conference, Quaker Square Hilton, Akron, Ohio, October
1-2, pp. 107-119 (1985), the disclosures of all which are
incorporated herein by reference. However, the preferred TREF
technique does not include purge quantities in SCBDI calculations.
More preferably, the monomer distribution of the interpolymer and
SCBDI are determined using .sup.13 C NMR analysis in accordance
with techniques described in U.S. Pat. No. 5,292,845 and by J. C.
Randall in Rev. Macromol. Chem. Phys., C29, pp. 201-317, the
disclosures of both of which are incorporated herein by
reference.
In addition to referring to a homogeneous (or narrow) short
branching distribution, the term "homogeneously branched linear
ethylene interpolymer" also means the interpolymer does not have
long chain branching. That is, the ethylene interpolymer has an
absence of long chain branching and a linear polymer backbone in
the conventional sense of the term "linear." However, the term
"homogeneously branched linear ethylene polymer" does not refer to
high pressure branched polyethylene which is known to those skilled
in the art to have numerous long chain branches. Homogeneously
branched ethylene polymers can be made using polymerization
processes (e.g., those described by Elston in U.S. Pat. No.
3,645,992) which provide a uniform (narrow) short branching
distribution (i.e., homogeneously branched). In his polymerization
process, Elston uses soluble vanadium catalyst systems to make such
polymers, however others such as Mitsui Chemical Corporation and
Exxon Chemical Corporation have used so-called single site catalyst
systems to make polymers having a similar homogeneous structure.
Homogeneously branched linear ethylene polymers can be prepared in
solution, slurry or gas phase processes using hafnium, zirconium
and vanadium catalyst systems. Ewen et al. in U.S. Pat. No.
4,937,299 describe a method of preparation using metallocene
catalysts. The disclosures of Elston and Ewen et al. are
incorporated herein by reference.
The term "heterogeneously branched linear ethylene polymer" is used
herein in the conventional sense in reference to a linear ethylene
interpolymer having a comparatively low short chain branching
distribution index. That is, the interpolymer has a relatively
broad short chain branching distribution. Heterogeneously branched
linear ethylene polymers have a SCBDI less than about 50 percent
and more typically less than about 30 percent.
Heterogeneously branched ethylene polymers are well known among
practitioners of the linear polyethylene art. Heterogeneously
branched ethylene polymers are prepared using conventional
Ziegler-Natta solution, slurry or gas phase polymerization
processes and coordination metal catalysts as described, for
example, by Anderson et al. in U.S. Pat. No. 4,076,698, the
disclosure of which is incorporated herein by reference. These
conventional Ziegler-Natta type linear polyethylenes are not
homogeneously branched, do not have any long-chain branching and,
as such, have a linear polymer backbone in the conventional sense
of the term "linear." At densities less than 0.90 g/cc, these
materials are more difficult to prepare than homogeneously branched
ethylene polymer and are also more difficult to pelletize than
their higher density counterparts. At such lower densities,
heterogeneously branched ethylene polymer pellets are generally
more tacky and have a greater tendency to clump together than their
higher density counterparts.
Typically, the homogeneously branched linear ethylene polymer and
the heterogeneously branched ethylene polymer are
ethylene/.alpha.-olefin interpolymers, wherein the .alpha.-olefin
is at least one C.sub.3 -C.sub.20 .alpha.-olefin (e.g., propylene,
1-butene, 1-pentene, 4methyl-1-pentene, 1-hexene, 1-octene and the
like) and preferably the at least one C.sub.3 -C.sub.20
.alpha.-olefin is 1-hexene. Most preferably, the ethylene/
.alpha.-olefin interpolymer is a copolymer of ethylene and a
C.sub.3 -C.sub.20 .alpha.-olefin, especially an ethylene/C.sub.4
-C.sub.6 .alpha.-olefin copolymer and most especially an
ethylene/1-hexene copolymer.
The term "substantially linear ethylene polymer" as used herein
refers to homogeneously branched ethylene/.alpha.-olefin
interpolymers that have a narrow short chain branching distribution
and contain long chain branches as well as short chain branches
attributable to homogeneous comonomer incorporation. The long chain
branches are of the same structure as the backbone of the polymer
and are longer than the short chain branches. The polymer backbone
of substantially linear (.alpha.-olefin polymers is substituted
with an average of 0.01 to 3 long chain branch/1000 carbons.
Preferred substantially linear polymers for use in the invention
are substituted with from 0.01 long chain branch/1000 carbons to 1
long chain branch/1000 carbons, and more preferably from 0.05 long
chain branch/1000 carbons to 1 long chain branches/1000
carbons.
Long chain branching is defined herein as a chain length of at
least 6 carbons, above which the length cannot be distinguished
using .sup.13 C nuclear magnetic resonance spectroscopy. The long
chain branch can be as long as about the same length as the length
of the polymer backbone to which it is attached. Long chain
branches are obviously of greater length than of short chain
branches resulting from comonomer incorporation.
The presence of long chain branching can be determined in ethylene
homopolymers by using .sup.13 C nuclear magnetic resonance (NMR)
spectroscopy and is quantified using the method described by
Randall (Rev. Macromol. Chem. Phys., C29, V. 2 & 3, p.
285-297).
As a practical matter, current .sup.13 C nuclear magnetic resonance
spectroscopy cannot determine the length of a long chain branch in
excess of six carbon atoms. However, there are other known
techniques useful for determining the presence of long chain
branches in ethylene polymers, including ethylene/ 1-octene
interpolymers. Two such methods are gel permeation chromatography
coupled with a low angle laser light scattering detector
(GPC-LALLS) and gel permeation chromatography coupled with a
differential viscometer detector (GPC-DV). The use of these
techniques for long chain branch detection and the underlying
theories have been well documented in the literature. See, for
example, Zimm, G. H. and Stockmayer, W. H., J. Chem. Phys., 17,
1301 (1949) and Rudin, A., Modem Methods of Polymer
Characterization, John Wiley & Sons, New York (1991) pp.
103-112.
A. Willem deGroot and P. Steve Chum, both of The Dow Chemical
Company, at the Oct. 4, 1994 conference of the Federation of
Analytical Chemistry and Spectroscopy Society (FACSS) in St. Louis,
Mo., presented data demonstrating that GPC-DV is a useful technique
for quantifying the presence of long chain branches in
substantially linear ethylene interpolymers. In particular, deGroot
and Chum found that the level of long chain branches in
substantially linear ethylene homopolymer samples measured using
the Zimm-Stockmayer equation correlated well with the level of long
chain branches measured using .sup.13 C NMR.
Further, deGroot and Chum found that the presence of octene does
not change the hydrodynamic volume of the polyethylene samples in
solution and, as such, one can account for the molecular weight
increase attributable to octene short chain branches by knowing the
mole percent octene in the sample. By deconvoluting the
contribution to molecular weight increase attributable to 1-octene
short chain branches, deGroot and Chum showed that GPC-DV may be
used to quantify the level of long chain branches in substantially
linear ethylene/ octene copolymers.
deGroot and Chum also showed that a plot of Log(I.sub.2, Melt
Index) as a function of Log(GPC Weight Average Molecular Weight) as
determined by GPC-DV illustrates that the long chain branching
aspects (but not the extent of long branching) of substantially
linear ethylene polymers are comparable to that of high pressure,
highly branched low density polyethylene (LDPE) and are clearly
distinct from ethylene polymers produced using Ziegler-type
catalysts such as titanium complexes and ordinary homogeneous
catalysts such as hafnium and vanadium complexes.
The substantially linear ethylene polymers used in the present
invention are a unique class of compounds that are further defined
in U.S. Pat. No. 5,272,236, Ser. No. 07/776,130, filed Oct. 15,
1991 and in U.S. Pat. No. 5,278,272, Ser. No. 07/939,281, filed
Sep. 2, 1992, each of which is incorporated herein by
reference.
Substantially linear ethylene polymers differ significantly from
the class of polymers conventionally known as homogeneously
branched linear ethylene polymers described, for example, by Elston
in U.S. Pat. No. 3,645,992, in that substantially linear ethylene
polymers do not have a linear polymer backbone in the conventional
sense of the term "linear." Substantially linear ethylene polymers
also differ significantly from the class of polymers known
conventionally as heterogeneously branched traditional Ziegler
polymerized linear ethylene interpolymers (for example, ultra low
density polyethylene, linear low density polyethylene or high
density polyethylene made, for example, using the technique
disclosed by Anderson et al. in U.S. Pat. No. 4,076,698, in that
substantially linear ethylene interpolymers are homogeneously
branched interpolymers. Substantially linear ethylene polymer's
also differ significantly from the class known as free-radical
initiated highly branched high pressure low density ethylene
homopolymer and ethylene interpolymers such as, for example,
ethylene-acrylic acid (EAA) copolymers and ethylene-vinyl acetate
(EVA) copolymers, in that substantially linear ethylene polymers do
not have equivalent degrees of long chain branching and are made
using single site catalyst systems rather than free-radical
peroxide catalysts systems.
Single site polymerization catalyst (for example, the
monocyclo-pentadienyl transition metal olefin polymerization
catalysts described by Canich in U.S. Pat. No. 5,026,798 or by
Canich in U.S. Pat. No. 5,055,438) or, more preferably, single site
constrained geometry catalysts (for example, as described by
Stevens et al. in U.S. Pat. No. 5,064,802) can be used to prepare
substantially linear ethylene polymers, so long as the catalysts
are used consistent with the methods described in U.S. Pat. No.
5,272,236 and in U.S. Pat. No. 5,278,272. Such polymerization
methods are also described in PCT/US 92/08812 (filed Oct. 15,
1992). However, the substantially linear ethylene polymers are
preferably made by using suitable constrained geometry catalysts,
especially constrained geometry catalysts as disclosed in U.S.
application Ser. Nos.: 545,403, filed Jul. 3, 1990; U.S. Pat. No.
5,132,380; U.S. Pat. No. 5,064,802; U.S. Pat. No. 5,153,157; U.S.
Pat. No. 5,470,993; U.S. Pat. No. 5,453,410; U.S. Pat. No.
5,374,696; U.S. Pat. No. 5,532,394; U.S. Pat. No. 5,494,874; U.S.
Pat. No. 5,189,192; the teachings of all of which are incorporated
herein by reference.
Suitable cocatalysts for use herein include but are not limited to,
for example, polymeric or oligomeric aluminoxanes, especially
methyl aluminoxane or modified methyl aluminoxane (made, for
example, as described in U.S. Pat. No. 5,041,584, U.S. Pat. No.
4,544,762, U.S. Pat. No. 5,015,749, and/or U.S. Pat. No. 5,041,585,
the disclosures of which are incorporated herein by reference) as
well as inert, compatible, non-coordinating, ion forming compounds.
Preferred cocatalysts are inert, non-coordinating, boron
compounds.
The polymerization conditions for manufacturing the substantially
linear ethylene polymers used in the present invention are
preferably those useful in the continuous solution polymerization
process, although the application of the present invention is not
limited thereto. Continuous slurry and gas phase polymerization
processes can also be used, provided the proper catalysts and
polymerization conditions are employed. To polymerize the
substantially linear polymers useful in the invention, the single
site and constrained geometry catalysts mentioned earlier can be
used, but for substantially linear ethylene polymers the
polymerization process should be operated such that substantially
linear ethylene polymers are indeed formed. That is, not all
polymerization conditions inherently make the substantially linear
ethylene polymers, even when the same catalysts are used. For
example, in one embodiment of a polymerization process useful in
making substantially linear ethylene polymers, a continuous process
is used, as opposed to a batch process.
The substantially linear ethylene polymer for use in the present
invention is characterized as having
(a) a melt flow ratio, I.sub.10 /I.sub.2, equal to or greater than
5.63,
(b) a molecular weight distribution, M.sub.w /M.sub.n, as
determined by gel permeation chromatography and defined by the
equation:
(c) a gas extrusion rheology such that the critical shear rate at
onset of surface melt fracture for the substantially linear
ethylene polymer is at least 50 percent greater than the critical
shear rate at the onset of surface melt fracture for a linear
ethylene polymer, wherein the substantially linear ethylene polymer
and the linear ethylene polymer comprise the same comonomer or
comonomers, the linear ethylene polymer has an I.sub.2, M.sub.w
/M.sub.n and density within ten percent of the substantially linear
ethylene polymer and wherein the respective critical shear rates of
the substantially linear ethylene polymer and the linear ethylene
polymer are measured at the same melt temperature using a gas
extrusion rheometer,
(d) a single differential scanning calorimetry, DSC, melting peak
between -30 and 150.degree. C., and
(e) a short chain branching distribution index greater than about
50 percent.
The substantially linear ethylene polymers used in this invention
are homogeneously branched interpolymers and essentially lack a
measurable "high density" fraction as measured by the TREF
technique (i.e., have a narrow short chain distribution and a high
SCBD index). The substantially linear ethylene polymer generally do
not contain a polymer fraction with a degree of branching less than
or equal to 2 methyls/1000 carbons. The "high density polymer
fraction" can also be described as a polymer fraction with a degree
of branching less than about 2 methyls/1000 carbons.
The substantially linear ethylene interpolymers for use in the
present 20 invention are interpolymers of ethylene with at least
one C.sub.3 -C.sub.20 .alpha.-olefin and/or C.sub.4 -C.sub.18
diolefin. Copolymers of ethylene and an (.alpha.-olefin of C.sub.3
-C.sub.20 carbon atoms are especially preferred. The term
"interpolymer" as discussed above is used herein to indicate a
copolymer, or a terpolymer, or the like, where, at least one other
comonomer is polymerized with ethylene or propylene to make the
interpolymer.
Suitable unsaturated comonomers useful for polymerizing with
ethylene include, for example, ethylenically unsaturated monomers,
conjugated or non-conjugated dienes, polyenes, etc. Examples of
such comonomers include C.sub.3 -C.sub.20 .alpha.-olefins as
propylene, isobutylene, 1-butene, 1-hexene, 4-methyl-1-pentene,
1-heptene, 1-octene, 1-nonene, 1-decene, and the like. Preferred
comonomers include propylene, 1-butene, 1-hexene,
4-methyl-1-pentene and 1-octene, and 1-octene is especially
preferred. Other suitable monomers include styrene, halo- or
alkyl-substituted styrenes, tetrafluoroethylene,
vinylbenzocyclobutane, 1,4-hexadiene, 1,7-octadiene, and
cycloalkenes, e.g., cyclopentene, cyclohexene and cyclooctene.
Determination of the critical shear rate and critical shear stress
in regards to melt fracture as well as other rheology properties
such as "Theological processing index" (PI), is performed using a
gas extrusion rheometer (GER). The gas extrusion rheometer is
described by M. Shida, R. N. Shroff and L. V. Cancio in Polymer
Engineering Science, Vol. 17, No. 11, p. 770 (1977), and in
"Rheometers for Molten Plastics" by John Dealy, published by Van
Nostrand Reinhold Co. (1982) on pp. 97-99. GER experiments are
performed at a temperature of about 190.degree. C., at nitrogen
pressures between about 250 to about 5500 psig using about a 0.754
mm diameter, 20:1 L/D die with an entrance angle of about
180.degree.. For the substantially linear ethylene polymers
described herein, the PI is the apparent viscosity (in kpoise) of a
material measured by GER at an apparent shear stress of about
2.15.times.10.sup.6 dyne/cm.sup.2. The substantially linear
ethylene polymer for use in the invention are ethylene
interpolymers having a PI in the range of about 0.01 kpoise to
about 50 kpoise, preferably about 15 kpoise or less. The
substantially linear ethylene polymers used herein have a PI less
than or equal to about 70 percent of the PI of a linear ethylene
interpolymer (either a conventional Ziegler-Natta polymerized
interpolymer or a linear homogeneously branched interpolymer as
described by Elston in U.S. Pat. No. 3,645,992) having an I.sub.2,
M.sub.w /M.sub.n and density, each within ten percent of the
substantially linear ethylene interpolymer.
An apparent shear stress versus apparent shear rate plot is used to
identify the melt fracture phenomena and quantify the critical
shear rate and critical shear stress of ethylene polymers.
According to Ramamurthy in the Journal of Rheology. 30(2), 337-357,
1986, above a certain critical flow rate, the observed extrudate
irregularities may be broadly classified into two main types:
surface melt fracture and gross melt fracture.
Surface melt fracture occurs under apparently steady flow
conditions and ranges in detail from loss of specular film gloss to
the more severe form of "sharkskin." Herein, as determined using
the above-described GER, the onset of surface melt fracture (OSMF)
is characterized at the beginning of losing extrudate gloss at
which the surface roughness of the extrudate can only be detected
by 40.times. magnification. The critical shear rate at the onset of
surface melt fracture for the substantially linear ethylene
interpolymers is at least about 50 percent greater than the
critical shear rate at the onset of surface melt fracture of a
linear ethylene interpolymer having essentially the same I.sub.2
and M.sub.w /M.sub.n.
Gross melt fracture occurs at unsteady extrusion flow conditions
and ranges in detail from regular (alternating rough and smooth,
helical, etc.) to random distortions. For commercial acceptability
and optimum sealant properties, surface defects should be minimal,
if not absent. The critical shear stress at the onset of gross melt
fracture for the substantially linear ethylene interpolymers used
in the invention, that is those having a density less than about
0.91 g/cc, is greater than about 4.times.10.sup.6 dynes/cm.sup.2.
The critical shear rate at the onset of surface melt fracture
(OSMF) and the onset of gross melt fracture (OGMF) will be used
herein based on the changes of surface roughness and configurations
of the extrudates extruded by a GER. Preferably, in the present
invention, the substantially linear ethylene polymer will be
characterized by its critical shear rate, rather than its critical
shear stress.
Substantially linear ethylene polymers also consist of a single
polymer component material and are characterized by a single DSC
melting peak. The single melting peak is determined using a
differential scanning calorimeter standardized with indium and
deionized water. The method involves about 5-7 mg sample sizes, a
"first heat" to about 140.degree. C. which is held for about 4
minutes, a cool down at about 10.degree./min. to about -30.degree.
C. which is held for about 3 minutes, and heat up at about
10.degree. C./min. to about 180.degree. C. for the "second heat".
The single melting peak is taken from the "second heat" heat flow
vs. temperature curve. Total heat of fusion of the polymer is
calculated from the area under the curve.
For substantially linear ethylene polymers having a density of
about 0.875 g/cc to about 0.91 g/cc, the single melting peak may
show, depending on equipment sensitivity, a "shoulder" or a "hump"
on the low melting side that constitutes less than about 12
percent, typically, less than about 9 percent, and more typically
less than about 6 percent of the total heat of fusion of the
polymer. Such an artifact is observable for other homogeneously
branched polymers such as EXACT resins and is discerned on the
basis of the slope of the single melting peak varying monotonically
through the melting region of the artifact. Such an artifact occurs
within about 34.degree. C., typically within about 27.degree. C.,
and more typically within about 20.degree. C. of the melting point
of the single melting peak. The heat of fusion attributable to an
artifact can be separately determined by specific integration of
its associated area under the heat flow vs. temperature curve.
The molecular weight distributions of ethylene .alpha.-olefin
polymers are determined by gel permeation chromatography (GPC) on a
Waters 150 C. high temperature chromatographic unit equipped with a
differential refractometer and three columns of mixed porosity. The
columns are supplied by Polymer Laboratories and are commonly
packed with pore sizes of 10.sup.3, 10.sup.4, 10.sup.5 and 10.sup.6
.ANG.. The solvent is 1,2,4-trichlorobenzene, from which about 0.3
percent by weight solutions of the samples are prepared for
injection. The flow rate is about 1.0 milliliters/minute, unit
operating temperature is about 140.degree. C. and the injection
size is about 100 microliters.
The molecular weight determination with respect to the polymer
backbone is deduced by using narrow molecular weight distribution
polystyrene standards (from Polymer Laboratories) in conjunction
with their elution volumes. The equivalent polyethylene molecular
weights are determined by using appropriate Mark-Houwink
coefficients for polyethylene and polystyrene (as described by
Williams and Ward in Journal of Polymer Science, Polymer Letters,
Vol. 6, p. 621, 1968) to derive the following equation:
In this equation, a=0.4316 and b=1.0. Weight average molecular
weight, M.sub.w, is calculated in the usual manner according to the
following formula: M.sub.j =(.SIGMA.w.sub.i (M.sub.i.sup.j)).sup.j
; where w.sub.i is the weight fraction of the molecules with
molecular weight M.sub.i eluting from the GPC column in fraction i
and j=1 when calculating M.sub.w and j=-1 when calculating
M.sub.n.
For the homogeneously branched substantially linear ethylene
polymer or homogeneously branched linear ethylene polymer used in
the present invention, the M.sub.w /M.sub.n is preferably less than
3.5, more preferably less than 3.0, most preferably less than 2.5,
and especially in the range of from about 1.5 to about 2.5 and most
especially in the range of from about 1.8 to about 2.3.
Substantially linear ethylene polymers are known to have excellent
processability, despite having a relatively narrow molecular weight
distribution (that is, the M.sub.w /M.sub.n ratio is typically less
than about 3.5). Surprisingly, unlike homogeneously and
heterogeneously branched linear ethylene polymers, the melt flow
ratio (I.sub.10 /I.sub.2) of substantially linear ethylene polymers
can be varied essentially independently of the molecular weight
distribution, M.sub.w /M.sub.n. Accordingly, the preferred ethylene
.alpha.-olefin polymer for use in the present invention is a
substantially linear ethylene polymer.
A preferred shrink film of the present invention will be further
characterized as having a compositional hexane extractive level of
less than 15 percent, preferably less than 10 percent, more
preferably less than 6, most preferably less than 3 percent based
on the total weight of the mixture.
Temperature rising elution fractionation (TREF) such as described
by Wild et al. can be used to "fingerprint" or identify the novel
mixtures of the invention.
Another preferred shrink film of the present invention will be
characterized by a Vicat softening point of at least 75.degree. C.,
preferably at least 85.degree. C., and more preferably at least
90.degree. C.
Another embodiment of the present invention is a method of making
an improved shrink film either as a monolayer film or as a shrink
control layer in a multilayer structure. The method of making a
multilayer structure comprising the shrink control layer can
include a lamination and coextrusion technique or combinations
thereof, or using the polymer mixture alone, and can also
specifically include blown film, cast film, extrusion coating,
injection molding, blow molding, thermoforming, profile extrusion,
pultrusion, compression molding, rotomolding, or injection blow
molding operations or combinations thereof.
The shrink film of the present invention can be made using
conventional simple bubble or cast extrusion techniques, however,
preferred film structures are prepared using more elaborate
techniques such as "tenter framing" or the "double bubble," "tape
bubble" or "trapped bubble" process. The double bubble technique is
described by Pahkle in U.S. Pat. No. 3,456,044, the disclosure of
which is incorporated by reference.
The polymer mixture used in the invention can be formed by any
convenient method, including dry blending the individual components
and subsequently melt mixing in a mixer or by mixing the components
together directly in a mixer (e.g., a Banbury mixer, a Haake mixer,
a Brabender internal mixer, or a single or twin screw extruder
including a compounding extruder and a side-arm extruder employed
directly down stream of a interpolymerization process.
The polymer mixture used in the invention (as well as the at least
one first ethylene polymer and the at least one second ethylene
polymer) can be formed in-situ via the interpolymerization of
ethylene and the desired alpha-olefin using a single-site
catalysis, preferably a single-site constrained geometry catalyst,
in at least one reactor and a single-site catalysis, preferably a
single-site constrained geometry catalyst, or a Ziegler-Natta type
catalyst in at least one other reactor. The reactors can be
operated sequentially or in parallel. An exemplary in-situ
interpolymerization process is disclosed in PCT patent application
Ser. No. 94/01052, incorporated herein by reference.
The polymer mixture used in the invention (as well as the at least
one first ethylene polymer and the at least one second ethylene
polymer) can further be formed by isolating component (A) and/or
component (B) from a heterogeneously branched ethylene polymer by
fractionating the heterogeneous ethylene polymer into specific
polymer fractions (or by isolating component (A) from a
homogeneously branched ethylene polymer by fractionating the
homogeneously ethylene polymer into polymer fractions), selecting
the fraction(s) appropriate to meet the limitations specified for
component (A) or component (B), and mixing the selected fraction(s)
in the appropriate amounts with the at least one first ethylene
polymer component (A) or the at least one second ethylene polymer
component (B). This method is obviously not as economical as the
in-situ polymerization described above, but can nonetheless be used
to obtain the polymer mixture used in the present invention as well
as the at least one first ethylene polymer and the at least one
second ethylene polymer.
However, regardless of how the polymer mixture, the at least one
first ethylene polymer or the at least one second ethylene polymer
is manufactured, the mixture or polymer will be considered a
homogeneously branched ethylene polymer or, alternatively, a
heterogeneously branched ethylene polymer based on the above
definitions of that refer to heterogeneous branching and
homogeneous branching (i.e., the SCBDI) and based on specific whole
composition analysis (such as, for example, ATREF results) rather
than fractional analysis or manufacturing technique.
Additives, such as antioxidants (e.g., hindered phenolics, such as
IRGANOX.TM. 1010 or IRGANOX.TM. 1076 supplied by Ciba Geigy),
phosphites (e.g., IRGAFOS.TM. 168 also supplied by Ciba Geigy),
cling additives (e.g., PIB), SANDOSTAB PEPQ.TM. (supplied by
Sandoz), pigments, colorants, fillers, and the like may also be
included in the shrink film of the present invention. Although
generally not required, the shrink film of the present invention
may also contain additives to enhance antiblocking, mold release
and coefficient of friction characteristics including, but not
limited to, untreated and treated silicon dioxide, talc, calcium
carbonate, and clay, as well as primary, secondary and substituted
fatty acid amides, release agents, silicone coatings, etc. Still
other additives, such as quaternary ammonium compounds alone or in
combination with ethylene-acrylic acid (EAA) copolymers or other
functional polymers, may also be added to enhance the anti-static
characteristics of the shrink film of the invention and permit the
use of the inventive shrink film in, for example, the heavy-duty
packaging of electronically sensitive goods.
The shrink film of the invention may further include recycled and
scrap materials and diluent polymers, to the extent that the
improved shrink film properties discovered by the Applicants is not
adversely affected. Exemplary diluent materials include, for
example, elastomers, rubbers and anhydride modified polyethylenes
(e.g., polybutylene and maleic anhydride grafted LLDPE and HDPE) as
well as with high pressure polyethylenes such as, for example, low
density polyethylene (LDPE), ethylene/acrylic acid (EAA)
interpolymers, ethylene/vinyl acetate (EVA) interpolymers and
ethylene/methacrylate (EMA) interpolymers, and combinations
thereof.
Biaxially oriented film structures are used for their enhanced
strength, barrier and/or shrink properties. Biaxially oriented film
structures find utility in various packaging and storage
applications for non-foodstuffs and food items such as primal and
subprimal cuts of meat, ham, poultry, bacon, cheese, etc. A
biaxially oriented film structure utilizing the shrink film of the
present invention may be a two to seven layer structure, with a
sealant layer composition (such as, for example, but not limited
to, another polymer mixture, at least one homogeneous branched
substantially linear ethylene polymer, at least one homogeneously
branched linear ethylene polymer, or at least one heterogeneously
branched ultra or very low density polyethylene), an outer layer
(such as, for example, another polymer mixture or at least one
heterogeneously branched linear low density or ultra-low density
polyethylene), and a core layer (such as a biaxially oriented
polypropylene homopolymer or vinylidene chloride polymer)
interposed between. Adhesion promoting tie layers (such as
PRIMACOR.TM. ethylene-acrylic acid (EAA) copolymers available from
The Dow Chemical Company, and/or ethylene-vinyl acetate (EVA)
copolymers, as well as additional structural layers (such as
Affinity.TM. polyolefin plastomers, Engage.TM. polyolefin
elastomers, both available from The Dow Chemical Company, ultra-low
density polyethylene, or blends of any of these polymers with each
other or with another polymer, such as EVA) may be optionally
employed.
Other layers of the multilayer structure include but are not
limited to barrier layers, and/or tie layers, and/or structural
layers. Various materials can be used for these layers, with some
of them being used as more than one layer in the same film
structure. Some of these materials include: foil, nylon,
ethylene/vinyl alcohol (EVOH) copolymers, polyvinylidene chloride
(PVDC), polyethylene terepthalate (PET), oriented polypropylene
(OPP), ethylene/vinyl acetate (EVA) copolymers, ethylene/acrylic
acid (EAA) copolymers, ethylene/methacrylic acid (EMAA) copolymers,
ULDPE, LLDPE, HDPE, MDPE, LMDPE, LDPE, ionomers, graft-modified
polymers (e.g., maleic anhydride grafted polyethylene), and paper.
Generally, the shrink film of the present invention may be a layer
in a multilayer structure which comprises from 2 to about 7
layers.
Cook-in packaged foods are foods which are prepackaged and then
cooked. The packaged and cooked foods go directly to the consumer,
institution, or retailer for consumption or sale. A package for
cook-in must be structurally capable of withstanding exposure to
cook-in time and temperature conditions while containing a food
product. Cook-in packaged foods are typically employed for the
packaging of ham, turkey, vegetables, processed meats, etc. Because
of the relatively high softening point to shrink response
characteristic of the inventive shrink film, the shrink film of the
present invention is well-suited for cook-in as well as hot-fill
packaging applications.
Description of Test Methods
Densities and density differentials are measured in accordance with
ASTM D-792 and are reported as grams/cubic centimeter (g/cc). The
measurements reported in the Examples below as overall densities
were determined after the polymer samples have been annealed for 24
hours at ambient conditions in accordance with ASTM D-792.
The density and weight percent of the first ethylene polymer
component (A) for Example manufactured by in situ polymerization
using two reactors can determined by an Analytical Temperature
Rising Elution Fractionation (ATREF) technique. The hardware and
procedures used for the ATREF technique have been previously
described, e.g., Wild et al, Journal of Polymer Science, Poly.
Phys. Ed., 20, 41(1982), Hazlitt, et al., U.S. Pat. No. 4,798,081
and Chum et al., U.S. Pat. No.5,089,321, the disclosures of which
are incorporated herein by reference. However, for the Examples
provided herein, polymer mixtures were all manufactured by melt
extrusion on a twin screw extruder.
Melt index measurements for the overall compositions and single
component examples was performed according to ASTM D-1238,
Condition 190.degree. C./2.16 kilogram (kg). Melt index is
inversely proportional to the molecular weight of the polymer.
Thus, the higher the molecular weight, the lower the melt index,
although the relationship is not linear. Melt index is reported as
g/10 minutes. Melt index determinations can also be performed with
even higher weights, such as in accordance with ASTM D-1238,
Condition 190.degree. C./10 kg, which is known as I.sub.10.
The term "melt flow ratio" as defined herein in the conventional
sense as the ratio of a higher weight melt index determination to a
lower weight melt index determination. For measured I.sub.10 and
I.sub.2 melt index values, the melt flow ratio is conveniently
designated as I.sub.10 /I.sub.2.
Vicat softening temperatures were measured in accordance with ASTM
D1525 and secant moduli were measured in accordance with ASTM D882
on slow-cooled compression molded samples.
The following examples are provided for the purpose of explanation
and are not intended to suggest any particular limitation of the
present invention.
EXAMPLES
Examples 1 and 3 and Comparative Examples 2 and 4
In an evaluation to discover the requirements for improved shrink
properties, a single component ethylene polymer and three different
ethylene polymer blends were evaluated. Table 1 lists the various
polymers evaluated and their properties (i.e., melt index, density,
Vicat softening point and description of first and second polymer
components and their density differential, where applicable).
TABLE 1
__________________________________________________________________________
Ratio Melt Density Vicat of First Second Index, Density
Differential Softening Example 1.sup.st /2.sup.nd Component
Component g/10 min. g/cc (g/cc) Temp., .degree. C.
__________________________________________________________________________
1 60/40 A F 0.82 0.9085 0.022 88.3 Comp. 2 60/40 B E 0.94 0.9067
0.050 80.7 3 40/60 A G 0.92 0.9075 0.014 87.1 Comp. 4 NA NA NA 0.81
0.9059 NA 84.4
__________________________________________________________________________
NA denotes not applicable.
Component Resin A was XU-59220.04, an experimental substantially
linear ethylene/1-octene copolymer having an I.sub.2 melt index of
about 0.88 g/10 minutes and a density of about 0.898 g/cc as
supplied by The Dow Chemical Company. Component Resin B was
AFFINITY.TM. CL 8003, a substantially linear ethylene/1-octene
copolymer having an I.sub.2 melt index of about 1 g/10 minutes and
a density of about 0.885 g/cc as supplied by The Dow Chemical
Company. Component Resin F was DOWLEX.TM. 2045, a linear low
density ethylene/1-octene copolymer having an I.sub.2 melt index of
about 1.0 g/10 minutes and a density of about 0.920 g/cc as
supplied by The Dow Chemical Company. Component Resin E was
DOWLEX.TM. 2038.68, a linear low density ethylene/l-octene
copolymer having an I.sub.2 melt index of about 1.0 g/10 minutes
and a density of about 0.935 g/cc as supplied by The Dow Chemical
Company. Component Resin G was ATTANE.TM. 4201, an ultra low
density ethylene/ 1-octene copolymer having an I.sub.2 melt index
of about 1.0 g/10 minutes and a density of about 0.912 g/cc as
supplied by The Dow Chemical Company.
Melting characterization of water quenched films of each resin was
done using a Perkin-Elmer DSC-7. The DSC was calibrated using
indium and water as standards. The water-quenched films were put in
an aluminum pan and the samples were heated from -30.degree. C. to
140.degree. C. at 10.degree. C./minute. The total heat of fusion
for each resins was obtained from the area under the curve. The
residual crystallinities at various temperatures were obtained
using the partial area method by dropping a perpendicular at those
temperatures wherein total crystallinity was taken by dividing the
heat of fusion by 292 Joules/gram.
The resins were extruded into 30-mil cast sheets and quick quenched
using a chilled roll. The melt temperature at the die was about
480.degree. F. (249.degree. C.) for each resin and the chill roll
temperature was about 75.degree. F. (24.degree. C.). The cast
sheets were oriented at their respective lowest orientation
temperature using a T. M. Long Biaxial stretcher (a tenter framer
stretcher). The initial dimensions of the cast sheets was 2
inches.times.2 inches and the draw ratio for the stretcher was set
at 4.5.times.4.5 and the stretching rate was 5 inches per second
(12.7 cm/s). The cast sheets were pre-heated in the stretcher for
about 4 minutes prior to stretching and hot air was deflected so as
not to impinge on the cast sheets directly (i.e., to avoid hot
spots in the cast sheets).
In this evaluation, the lowest orientation temperature was taken as
the temperature that gave a percent residual crystallinity of about
20 percent which was approximately 5.degree. C. above the
temperature where the cast sheet would tear, show "banding" (i.e.,
uneven deformation) or would repeatedly dislodged itself from the
grips of the stretcher during stretching at a grip pressure of
about 500 psi. The orientation window was taken as the temperature
range from the lowest orientation temperature to the highest DSC
peak melting temperature of the sample.
The oriented cast sheets were tested for unrestrained (free) shrink
at 90.degree. C. by measuring unrestrained shrink in a water-bath
at 90.degree. C. The samples were cut 12 cm.times.1.27 cm. The
samples were marked with a marker exactly 10 cm. from one end for
identification. Each sample was completely immersed in the water
bath for five seconds and then quickly removed. Film shrinkage was
obtained from the calculations in accordance with ASTM D-2732-83
and were taken from the average of four samples.
Table 2 summarizes the secant modulus, shrink response and
orientation temperature for Examples 1 and 3 and comparative
examples 2 and 4:
TABLE 2
__________________________________________________________________________
Weight % 2% Percent Shrink @ Crystallinity @ Orientation Secant
90.degree. C. Orientation Orientation Window Example Modulus (hot
H.sub.2 O) Temp C. Temp .degree. C.
__________________________________________________________________________
1 17,023 34.5 87.8 20.8 33 Comp. 2 17,218 25.0 93.3 19.6 29 3
15,327 30.8 87.8 21.0 34 Comp. 4 12,832 26.0 90.6 19.9 30
__________________________________________________________________________
The data in Table 2 indicate that Examples 1 and 3 are optimized
shrink films relative to comparative examples 2 and 4. Examples 1
and 3 exhibited the highest shrink responses and broadest
orientation windows. Inventive Example 3 exhibited a shrink
response at least 18 percent higher than the single component
heterogeneously branched linear ethylene polymer and Inventive
Example 1 exhibited a shrink response at least 32 percent higher
than the single component heterogeneously branched linear ethylene
polymer. Additionally, Table 1 above indicates that Examples 1 and
3 also exhibited the highest softening temperature relative to
comparative examples 2 and 4.
Examples 5 and 7 and Comparative Examples 6, 8 and 9
In another evaluation, another single component ethylene polymer
and four different ethylene polymer blends were evaluated to
discover the requirements for improved shrink properties at higher
polymer densities. Table 3 lists the various polymers evaluated and
their properties (i.e., melt index, density, Vicat softening point
and description of first and second polymer components and their
density differential, where applicable).
TABLE 3
__________________________________________________________________________
Ratio Melt Density Vicat of First Second Index, Density
Differential Softening Example 1.sup.st /2.sup.nd Component
Component g/10 min. g/cc (g/cc) Temp., .degree. C.
__________________________________________________________________________
5 40/60 D F 1.0 0.914 0.018 96 Comp. 6 60/40 C E 1.28 0.9133 0.0385
91.5 7 30/70 A F 0.86 0.9146 0.022 96 Comp. 8 60/40 A E 0.85 0.9141
0.037 94 Comp. 9 NA NA NA 0.92 0.9128 NA 95.8
__________________________________________________________________________
NA denotes not applicable.
Component Resin A was XU-59220.04, an experimental substantially
linear ethylene/1-octene copolymer having an I.sub.2 melt index of
about 0.88 g/10 minutes and a density of about 0.898 g/cc as
supplied by The Dow Chemical Company. Component Resin C was
AFFINITY.TM. PF 1140, a substantially linear ethylene/1-octene
copolymer having an I.sub.2 melt index of about 1.6 g/10 minutes
and a density of about 0.8965 g/cc as supplied by The Dow Chemical
Company. Component Resin D was AFFINITY.TM. PL 1880, a
substantially linear ethylene/1-octene copolymer having an l.sub.2
melt index of about 1.0 g/10 minutes and a density of about 0.902
g/cc as supplied by The Dow Chemical Company. Component Resin E was
DOWLEX.TM. 2038.68, a linear low density ethylene/1-octene
copolymer having an I.sub.2 melt index of about 1.0 g/10 minutes
and a density of about 0.935 g/cc as supplied by The Dow Chemical
Company. Component Resin F was DOWLEX.TM. 2045A, a linear low
density ethylene/1-octene copolymer having an I.sub.2 melt index of
about 1.0 g/10 minutes and a density of about 0.920 g/cc as
supplied by The Dow Chemical Company.
The test methods and procedures used for Examples 5 and 7 and
comparative examples 6, 8 and 9 were the same for Examples 1,
except instead of a water-bath to induce shrinkage, hot oil at
105.degree. C. was used and the orientation temperature was taken
at approximately 21% residual crystallinity rather than at
approximately 20%. Table 4 summarizes the various results.
TABLE 4
__________________________________________________________________________
Weight % 2% Percent Shrink @ Crystallinity @ Orientation Secant
105.degree. C. Orientation Orientation Window Example Modulus (hot
H.sub.2 O) Temp C. Temp .degree. C.
__________________________________________________________________________
5 21,683 44.5 96.1 20.6 25 Comp. 6 21,593 35.8 97.8 21.8 23 7
23,692 41.3 96.1 22.5 25 Comp. 8 24,204 37.8 98.3 21.9 23 Comp. 9
18,770 38.5 98.3 21.0 23
__________________________________________________________________________
The data in Table 4 indicate that Examples 5 and 7 are optimized
shrink films relative to comparative examples 6, 8 and 9. Examples
5 and 7 exhibited the highest shrink responses and equivalent to
broader orientation windows. Further, Table 3 above indicates that
Examples 5 and 7 also exhibited the highest softening temperature
relative to comparative polymer blends, comparative examples 6 and
8.
* * * * *